FreeCodeCamp JS interview studying

Question 1

var

Answer 1

• var is functionally or globally scoped
• can be redeclared
• can be redeclared without initialization
• can be updated

Question 2

let

Answer 2

• let is block scoped
• cannot be re-declared within it’s scope.
• can be declared without initialization
• can be updated

Question 3

const

Answer 3

• const is block scoped
• cannot be re-declared within it’s scope
• must be initialized at the time of declaration
• can never be updated

Question 4

==

Answer 4

only checks for the value. performs coercion before checking.

Question 5

===

Answer 5

checks for value + type. no coercion.

Question 6

what are the most frequently used array methods in JS?

Answer 6

map, filter, find, reduce, forEach

Question 7

The map() array method

Answer 7

• map creates a new copy of the original array
• use when you want to do something with elements without changing the OG array
• iterates over OG array and takes a callback fn as an arg.
• map tells it what to do with each element

Question 8

The filter() array method

Answer 8

• filter creates a NEW array with elements that meet the given condition(s)

Question 9

the forEach() array method

Answer 9

• similar to map() with 2 key differences.
• unlike map(), forEach() does NOT return a new array
• you cannot do method chaining with forEach()
• does NOT mutate the original array

Question 10

What are the two ways to make a function in JS?

Answer 10

1. normal function
2. arrow function

Question 11

What are the 3 types of scope?

Answer 11

• Global (declaration outside of any function)
• Function (declaration inside a function)
• Block (declaration inside a block)

Question 12

What is a closure?

Answer 12

An inner function returned by an outer function that references a variable defined in the outer function, ie. a “closed-over-variable”

Even when functions are returned (in the above case y) they still remember their lexical scope (where it came from)

So, when we finally invoke z, y is invoked. Now, y has to log a so it first tries to find 🔍 it in the local memory but it’s not there. It goes to its parent function. It finds a there.

Question 13

What is a lexical environment? How is it related to a closure?

Answer 13

It is essentially the surrounding state – the local memory along with the lexical environment of its parent.

Question 14

What are the advantages of closures in JavaScript?

Answer 14

• Currying
• Data hiding/encapsulation

Question 15

What are the disadvantages of closures in JavaScript?

Answer 15

Overconsumption of memory or memory leaks can happen.

For example, the closed-over-variable will not be garbage collected. This is because, even if the outer function has run, the returned inner function still has a reference to the closed-over-variable.

Note: Garbage collection basically removes unused variables from the memory automatically.

Question 16

What is hoisting in JavaScript?

Answer 16

This is JavaScript’s default behavior of moving declarations to the top of the program.

Question 17

How is var hoisted in JS?

Answer 17

var declaration is hoisted up and initialized with undefined.

Question 18

How are let and const hoisted in JS?

Answer 18

let and const declarations are hoisted up but not initialized.

Question 19

how are function definitions hoisted in JS?

Answer 19

function definitions are also hoisted up and stored as they are.

Question 20

What is Implicit Binding?

Answer 20

When the value of this depends on how and where we are doing the calling.

Question 21

What is Explicit Binding?

Answer 21

Explicit binding is when you force a function to use a certain object as its this.

Question 22

call()

Answer 22

• for explicit binding
  execution: runs instantly
  parameter: list of items

Question 23

apply()

Answer 23

• for explicit binding
  execution: runs instantly
  parameter: array []

Question 24

bind()

Answer 24

• explicit binding
  execution: assign, use later
  parameter: list of items

Question 25

What are some unique qualities of arrow functions?

Answer 25

• this … the value depends on the lexical scope (the outer function where the arrow fn is defined)
• arrow functions inherit the parent’s context

Question 26

Prototypes and Prototypal Inheritance in JavaScript

Answer 26

Whenever we create anything (like an object or function) in JavaScript, the JS Engine automatically attaches that thing with some properties and methods.

All this comes via prototypes.

__proto__ is the object where JS is putting it all.

Question 27

What is Prototypal Inheritance in JavaScript?

Answer 27

Question 28

What is the setTimeout() method?

Answer 28

The setTimeout() method calls a function or evaluates an expression after a specified number of milliseconds.

use clearTimeout to stop

Question 29

What is the setInterval() method?

Answer 29

The setInterval() method calls a function or evaluates an expression after a specified interval,

use clearInterval to stop the timer

Question 30

Promises in JS

Answer 30

• at the heart of async js
• can be 1 of 3 states (pending, fulfilled, rejected)
• (resolve/reject) are conventional names, can be anything
• advantage of cleaner syntax to avoid “callback hell”

Question 31

Polyfills in JS

Answer 31

A polyfill is a piece of code (usually JavaScript on the Web) used to provide modern functionality on older browsers that do not natively support it. MDN

Question 32

async vs defer in JS

Answer 32

• Async and defer are boolean attributes which can be loaded along with the script tags. They are useful for loading external scripts into your web page.

If there are multiple scripts which are dependant on each other, use defer. Defer script are executed in the order which they are defined.

If you want to load external script which is not dependant on the execution of any other scripts, use async.

Note: The async attribute does not guarantee the order of execution of scripts.

Question 33

Debounce vs Throttling in JS

Answer 33

So, to summarize, debouncing is when the difference between two keyup events is 300 ms.

And throttling is when the difference between two function calls is 300 ms. Basically, the function is called after a certain interval of time.

Question 34

localStorage in JS

Answer 34

localStorage: Data persists even after closing your session

Question 35

sessionStorage

Answer 35

sessionStorage: You lose your data when your session is over, like when you close the browser on the tab.